A satellite communication system typically includes a satellite and a plurality of earth stations that communicate with one another by way of the satellite which receives a signal from a sending earth station and transmits the signal to a receiving earth station. Generally, the system includes a control processor which serves to control the time at which any one station is permitted to send since the satellite is time-shared by a number of earth stations. Typically, the control processor is located on earth although it can be located on the satellite.
Various protocols have been used in the past to control the times at which each station is permitted to send.
One basic protocol is to allow random access in a contention process. The first station to seize control when the system is otherwise idle is allowed to transmit, either as long as it wishes, or for a prescribed interval. However, such a system is not efficient during periods when many stations are seeking to transmit at about the same time. A number of variants of this contention type protocol have been proposed to improve this basic approach but each variant tends to retain the fundamental problem that in heavy traffic some stations may experience difficulty in getting any access to send in a reasonable time.
Another basic protocol depends on a reservation process in which each station is allotted a reserved period in which it alone can send. This protocol is inefficient when many of the stations have so little to send much of the time that most of their reserved periods are wasted.
Recently, there have been proposed protocols which combine features of the basic contention protocol and the basic reservation protocol. A system of this kind of particular promise is described in U.S. Pat. No. 4,742,512 which issued on May 3, 1988 to F. Akashi et al.
In this system, polling signals are transmitted by way of the satellite to each earth station in turn on the broadcast channel from a control center that is responsible for controlling access to the satellite, and begins by polling cyclically each station for requests for reservations without awaiting a reply. A station requesting a reservation transmits its request to the control center on the multiple access satellite channel. A second polling signal is then transmitted on the satellite broadcast channel from the control center to each requesting station in response to the reservation request and the multiple access satellite channel is reserved for a set period of time for the requesting station. Thereafter, at the appropriate time set by the reservation signal, the requesting station transmits to the satellite packets of information for the allotted time. While this mixed protocol offers significant advantages in both light traffic and heavy traffic conditions over pure reservation protocols, it still suffers inefficiencies when the traffic is quite nonuniform because it uses a "fixed window" scheme in which each station requesting a reservation is offered the same maximum so that an extremely heavy user gets no better treatment than a merely heavy user.
The present invention seeks to improve the above described system.